Optical isolators are used in fiber optic systems to propagate signals in a direction while preventing propagation of signals along a reverse of the direction. These optical isolators are often used to prevent unwanted signal reflections from being transmitted back to a signal's source. Typical optical isolators have components to separate, rotated, and recombine orthogonally polarized light. These components function to propagate light beams from the input to the output of the isolator while diverting light beams traveling in the opposite direction to prevent them from reaching the input port.
One type of optical isolator uses an isolator core positioned in-line between two lens to couple light from an input fiber to an output fiber. The isolator core has, in sequence, an input walk-off crystal, a Faraday rotator, a half-wave plate, and an output walk-off crystal. In the forward direction, the input walk-off crystal divides an incident light beam from the input fiber into two polarized rays that are rotated by the Faraday rotator and the half-wave plate and then recombined by the output walk-off crystal. The Faraday rotator is a non-reciprocal device that rotates the polarization of light rays in the same direction regardless of which direction rays are passed through the device.
In the reverse direction, the output walk-off crystal also divides a light beam into two polarized rays. However, the non-reciprocal function of the Faraday rotator prevents the polarization of light traveling in the reverse direction from being rotated back to a position that will allow the two rays to be recombined by the input walk-off crystal. In this manner, light is prevented from reaching the input fiber in the reverse direction through the isolator core.
One problem with such an optical isolator is that number and size of individual components adds to the overall length and size of the optical isolator, thereby decreasing its desirability.